The mechanism is a countercurrent heat exchanger called a rete mirabile. Warm venous blood leaving the red swimming muscle runs alongside cold arterial blood arriving from the gills, and heat crosses between them before it can be lost across the gill surface. The result is that muscle, stomach and brain stay warmer than ambient water rather than tracking it.
Measured differences in white sharks are usually a few degrees Celsius above ambient, with the largest gradients in the coldest water. The same anatomy appears in the other lamnid sharks and in tunas, which is a textbook case of convergent evolution rather than shared ancestry.
The common error is to call the shark warm-blooded in the mammalian sense. A great white does not hold a fixed set point and cannot warm itself when still; the heat is a by-product of swimming that the vascular plumbing declines to throw away. How much of the advantage is faster muscle power versus a longer digestive window is still argued.